In a direct laser deposition process, a structure is formed directly onto a substrate. The structures are typically metallic and generated by depositing a metal powder into a melt pool formed on the substrate by heat from a laser that traverses over the substrate. The deposited powder melts in the melt pool and as the laser moves from the location to which the metal powder has been directed the deposit cools and solidifies to form a structure with a height. By making repeated passes over the same location, it is possible to increase the height of the formed structure.
Powder delivery nozzles are typically either coaxial with the laser with the powder focussing nozzle being conical around the beam optics, or arranged to the side of the laser with the powder being fed across the beam path. The powder is fed to the melt pool within an inert gas stream, usually argon. The flow characteristics of the powder flow can vary with time. Ballistic scatter, powder behaving as fume, powder collecting as “snowdrift” or “cornice” in the feed system, condensation, electrostatic agglomeration, contamination or mechanical interlocks caused by semi-fused satellite particles can all cause variation in flow rate. Similarly, as the CNC system moves or changes orientation the focal position of the powder stream can vary and ballistic scatter, caused by a variation in the particle size for example, can cause skew in the powder density distribution, which would vary with angle and stand-off distance from the work piece. The particles are also abrasive and can wear the powder feed nozzles to generate a larger orifice with a corresponding larger particle distribution and reduced focus. Powder feed nozzles can also clog with fume (metal vapour condensate), as well as suffering distortion from stray beam impingement and/or from radiant and reflected.
If the powder flow pattern is not consistent, the formed structure may not be uniform.